What is a Bolted Connection?
Bolted connection is a method to join structural components with the help of bolts. Bolted connections are more common than other types of connections used for forming structural joints. They're simple to use and don't require any extra equipment. This is owing, in part, to the development of higher-strength fasteners, which have made it possible to create simple and strong structural steel connections.
The process for establishing a bolted connection involves the following steps:
- Members to be connected are overlapped.
- Bolt holes are drilled on the overlapped portion.
- Bolts are inserted into the hole with washers at their position.
- Nuts are inserted in the shank portion and tightened with the help of spanners.
- Due to the tightening of the nut, clamping action is developed in the bolts, which are transferred to plates with the help of washers.
Classifications of Bolted Connection
The bolted connection can be classified in various categories on the basis of the line of action of resultant force transferred, on the basis of force experienced by the bolts, or on the basis of the force transfer mechanism.
Types of Bolted Connections
Based on the line of action of resultant force transferred, bolted connections can be divided into two categories:
- Concentric connections
- Eccentric connections
When the line of action of the resultant force passes through the center of gravity of the bolted connection, then it is called a concentric connection. When the line of action of the resultant force does not pass through the centroid of the bolted connection, then it is known as an eccentric connection.
Based on the type of force experienced by the bolts, the bolted connections can be categorized into three categories -
- Tension connections,
- Shear connections, and
- Combined shear and tension connections.
As the name suggests, it is known as tension connection when bolts are subjected to tension. In shear connection, the bolts are subjected to shear; in combined shear and tension, bolts are subjected to both tension and shear.
Based on the type of force transfer mechanism, bolted connections can be divided into two categories:
- Bearing connection and
- Friction mechanism
In bearing connection, ordinary bolts with low strength are used, so the amount of clamping action that can be generated by tightening is small. Thus friction between connected elements is negligible. Therefore, when the load is applied, a slip occurs. Then bolt and connecting members come in contact. Load is transferred to bolts from members through contact, known as Bearing.
In the friction mechanism, high-strength friction grip bolts are used in connections. HSFG bolts transfer load by friction. These bolts have high strength, so they can be tightened up to the desired degree. Thus they are tightened such as to induce a predefined tension in the bolt. The tightening of bolts generates a clamping action in which the members are connected.
Tension in the bolt's interface between the members generates a frictional resistance towards applied loading due to tension. The applied load is transferred by friction between the members.
Types of Joints in Bolted Connections
- Lap Joint: Two members to be connected are overlapped and joined. The load lines in 2 members of the lap joint do not coincide; hence the lap joint has an eccentricity leading to the formation of an undesirable bending. This will develop tension in the bolt, which may lead to the failure of the bolt.
- Butt joint: Members to be connected are placed end to end, thereby bringing load lines in the 2 members in one line. Additional plates are used on either one or both sides of the main plates for connecting the main plates, which are placed end to end.
Load in a lap connection has an eccentricity as the CG of load in one connected member is not lying with the CG of a load in another connected member, which causes undesirable bending. The double cover butt connection is free from moments (zero eccentricity). The shear strength of each bolt in a double cover butt connection is two times higher than the shear strength of each bolt in a lap connection (or) single cover butt connection.
Types of Failure in Bolted Connection
Bolted connections can fail either due to failure of the connection itself or due to failure of the connecting components. Some of these failures, like the plate's shear failure, the plate's splitting failure, and the plate's bearing failure, can be prevented by adhering to edge distance criteria as mentioned in IS 800: 2007. As a result, they are irrelevant, whereas the others deserve careful thought. The bolted connection can fail in any of the six methods listed below.
- Tension failure of the plate occurs when the bolts are stronger than the connected plates.
- Bearing failure of bolt occurs when low strength bolts are used to connect high strength plates. In this failure, the bolt gets crushed around half of its circumference when the connected plates slip due to applied force.
- Shear failure of a bolt, the bolt gets cut or separated about the shear interface. The number of shear interfaces can be one or multiple depending upon joints. Based on the number of shear planes, a bolt may fail in Single shear, Double shear, etc. Shear stresses are created when plates slip due to applied forces. If the maximum factored shear force exceeds the shear capacity of bolts, the bolt's shear failure occurs at the bolt shear plane.
- Shear failure of a plate is a type of excessive bearing failure when the bolt hole is near the end of the plate.
- Plate splitting failure occurs when high-strength bolts are used to connect high-strength plates. It is a combined failure of shear and tension. Sometimes bolts may have to be placed at a lesser end distance than required, leading to plates shearing out. Due to this, a block of material within the bolted area breaks away from the remaining area.
- Bearing failure of the plate also occurs when high-strength bolts are used to connect low-strength plates. The presence of a neighboring bolt or the proximity of an edge in the load direction can aggravate the bearing problem. When ordinary bolts are subjected to shear forces, slip occurs, and bolts come in contact with the plates. The plate may get crushed if the plate material is weaker than the bolt material.
Common Terms in Bolted Connection
Before understanding the design of bolted connections and their specifications, certain terminologies involved in these connections must be understood. These common terms which will be used throughout the design of connections are listed below:
- Gauge distance: It is the centre to centre distance between two consecutive bolts measured in a perpendicular direction to the applied forces/stresses.
- Pitch distance is defined as the centre to centre distance between two consecutive bolts measured along a row of bolts. A row generally refers to a line of bolts placed parallel to the direction of load in a member.
- Edge distance: This is the distance perpendicular to the direction of stress from the centre of the bolt hole to the adjacent edge of the member.
- End distance: This is the distance in the direction of stress from the centre of a bolt hole to the end of the element.
- Nominal diameter of bolt: It is the diameter of the shank portion of the bolt. It is determined by Unwin’s Formula.
- Diameter of bolt hole: It is the diameter of holes in which bolts are driven.
Specifications for Bolted Connection as per IS 800
IS 800: 2007 has defined certain specifications for the bolted connection for a safe design. The design engineer needs to follow these set standards to avoid any failures of the bolted connections, as mentioned above in this article. These specifications are as follows:
- Nominal diameter of bolt:It is estimated using Unwin’s formula, d=6.04√t, where t is the thickness of the plate.
- Diameter of bolt hole: It is obtained by adding a nominal diameter of a bolt with the clearance provided.
Nominal diameter of bolt (mm)
Diameter of Bolt hole (mm)
12 - 14
13 - 15
16 - 24
18 - 26
- Pitch distance: Minimum pitch distance should be 2.5d, where d is the nominal diameter of the bolt. The maximum pitch distance for tension members is 16t or 200 mm, whichever is the minimum. For compression members, the maximum pitch distance is 12t or 200 mm, whichever is the minimum. Here, t is the thickness of the thinner member. In the case of staggered bolting, the above values for maximum pitch should be increased by 50%, given that the gauge distance is not greater than 75 mm.
- Edge and End distance: Minimum edge and end distance should be 1.7do for hand-cut bolts and 1.5do for machine-cut bolts. Here, do is the diameter of the bolt hole. Maximum end and edge distance are given by 12t, where Ε=√250/fy and t is the thickness of the thinner member.
Advantages of Bolted Connection
Every steel design aims to ensure that the parts are properly attached to one another. Hence, connection designers are guided by three factors when creating joints that do the job: cost-effectiveness, safety, and effectiveness. Here's a rundown of the benefits of bolted connections:
- Facilitate faster erection of the structure.
- Skilled laborers are not required.
- Reducing skilled labor and equipment costs is more economical than riveted connections.
- Fabrication is simple. Technicians can easily reproduce the bolt you already have in hand and create additional for your needs because they are simple to fabricate.
- Once the bolts are in place and tightened, the load is immediately supported, regardless of how heavy or light it is.
- Over time, the bolted connection shows relatively little deformation than riveted and welded connections.
Disadvantages of Bolted Connection
Engineers must pick whether material or tool is ideal to fulfill various tasks throughout any project's planning and design stage. The types of joints used in a structure are normally determined by the overall design, but cost, installation time, and overall performance are the factors that also need to be considered by the design engineer. Depending upon these additional factors, there are several disadvantages of bolted connections, such as,
- High cost of material compared to rivets.
- Due to the reduction in the area of the bolt in the threaded portion, the tensile strength of the bolt decreases, and stress concentration occurs.
- In the case of dynamic loads and shock loads, bolted connections get loosened up.